Trommel Fines Feedstock
Trommel fines feedstock was supplied by Biffa Ltd, a UK commercial waste management company based in Leicester. The samples originated from the recycling of household wastes, following a mechanical processing step to remove majority of the metals, glass, and plastics material. In a typical process, the remaining materials are crushed or shredded and passed from trommel screens for size classification. The ‘unders’ which represent particle sizes less than 25 mm are then classed as trommel fines [16, 17]. At Biffa Ltd, the ‘unders’ are further passed through a 10 mm screen to reduce the heterogeneity of the sample, which was used in this present study. Further pre-treatment of the ‘as-received’ feedstock was carried out to ensure that the sample met the particle size requirements of the bubbling fluidised bed reactor used in this study according to the procedure described in an earlier paper [7]. Briefly, the feedstock was bone-dried to 2.69 wt% moisture content in an oven at 60 °C for 24 h, sieved and physically separated by manual removal of large visible stones, glasses, concrete and bones. The remaining material was then ground using a Retsch Ltd., Germany, Heavy-Duty Cutting Mill, Knife Mill Type SM2000. The ground sample was again sieved to obtain a final prepared feedstock, with particle size range of 0.5–2 mm. This fraction accounted for approximately 70 wt% and more than 80% of the energy content (calorific value) of the original feedstock [7]. Table 1 summarises the main characteristics of this fraction. Thermogravimetric analysis (TGA) of the feedstock is shown in Fig. 1 [7]. In order to investigate the effect of moisture content on the fast pyrolysis of the feedstock, two additional samples with nominal moisture contents of 5 wt% and 10 wt% were prepared, respectively. In the preparation, the bone-dry feedstock (2.69 wt%, moisture) was wetted with the required amount of water and left in a cool, dark place for 72 h before use. Just before pyrolysis, the actual moisture contents of the feedstocks were determined again and reported in Table 2.
Table 1 Characteristics of prepared bone-dry trommel fines feedstock (0.5–2 mm) [7]
Table 2 Product yields and mass balance summary from fast pyrolysis of trommel fines in relation to feedstock moisture contents
Fast Pyrolysis Rig
Fast pyrolysis experiments were carried out in an existing 300 g h−1 bench-scale bubbling fluidized bed reactor unit, which is shown in Fig. 2. In a brief description, it consists mainly of a dome-bottomed cylindrical hopper-feeder system, a reaction (fast pyrolysis) chamber and a product collection system. The feeder uses a dual screw gravimetric feeding system with variable speed motor for feeding, attached to a fast screw. The pyrolysis chamber is a tubular steel reactor having 41 mm internal diameter and a height of 320 mm. About 150 g of calcined quartz sand of particle size range of 500–600 µm was used as bed material, fluidized by a stream of nitrogen gas (99.9% purity) at an optimized flow rate of 6 L min−1 for heat transfer [18, 19]. The product collection system consisted of a water-cooled condenser and two dry ice/acetone-cooled condensers, followed by a cotton wool filter.
Fast Pyrolysis Experiments
Each experiment was initiated by starting the inert nitrogen flow for 10 min to remove the air within the system, followed by preheating the reactor using an electrical furnace. The fast pyrolysis temperature was maintained at approximately 500 °C, by setting the heater temperature at 50 °C above the reactor temperature throughout the duration of each run. Once the temperatures of the fluidising medium reached a steady state, the prepared trommel fines feedstock was continuously fed into the reactor at the middle of the fluidized bed by nitrogen entrained flow via the air-cooled feeding tube (Fig. 2). The feeding rate was set at an average of 170 g h−1 and each experiment lasted for 1 h. After pyrolysis in the reactor, the pyrolysis vapours including aerosols, water and non-condensable gases, were carried by the nitrogen gas stream through a cyclone to remove entrained solids, which were collected in the char pot. The ensuing vapour stream then passed through the system of condensers, where the condensable components were collected as liquids. Two sets of liquid products were collected each time; the first from the water-cooled condenser and the second from the two dry ice/acetone condensers, respectively. Any escaping vapour was then mostly captured in the cotton wool filter, before the non-condensable gases directed into a gas meter, where the volume of the exit gas was recorded. A portion of the exit gases was taken by an automatic sampling system into an automated online gas chromatograph (GC) for gas composition analysis, while the remainder was vented through an installed ventilation system.
Characterization of Fast Pyrolysis Products
For each moisture content variable, fast pyrolysis experiment was conducted five times and the resulting mass balances were compared to select the three tests with the closest yields of pyrolysis products, i.e. within standard deviations of < 5% of each other. These were then used to compute the average product mass balance closures reported in this work [18, 19]. Detailed characterisations of the gas products and solid residues obtained from the three selected experiments were carried out in triplicates and average results reported with the standard deviations. However, GC/MS analyses were carried out only the liquid products from the experiment with the highest mass balance closure for each set of moisture content experiments. Similarly, XRF elemental analyses (in triplicates) were carried out only on the bone-dried trommel fines feedstock and its solid residue from the test with the highest mass balance closure.
Analysis of Gas Products
During a pyrolysis run, the non-condensable gases were sampled every 3 min into a Micro-gas chromatograph with a thermal conductivity detector (TCD) from Varian Chromatography System Inc. The gas components were separated on two columns (Varian CP-5A Molsieve and CP-PortaPLOT) prior to detection. Quantitation was achieved by external standard method by calibrating the detector response using a standard gas mix containing hydrogen, oxygen, carbon monoxide, carbon dioxide and C1–C4 hydrocarbon gases at 3 vol% concentrations in nitrogen. The mass yields of the gas components calculated using the general gas equation, based on the gas volume composition obtained from GC analysis, total gas volume and the exit gas temperature and recorded pressure.
Characterization of Liquid Products
Each liquid product was found to be composed of both an aqueous and an organic fraction. The first-condensate liquid products also contained some solids. Further analysis of the organic fraction was carried out using GC–MS to determine the main organic compounds present. Elemental analysis and heating values of the liquid products were also determined.
Volumetric Karl-Fischer (KF) titration was used to determine the water content of all the fast pyrolysis primary and secondary condensates [20]. The primary and secondary condensates were dissolved in a known amount of acetone (1:6) prior to analyses. The dilution with acetone served two purposes; (a) to form a miscible solution of pyrolysis oil and water in acetone; (b) to dilute the samples and adjust the pH of the solution and water contents to the optimum ranges for Karl-Fischer titration of between 5 and 7 and < 50 wt%, respectively [21, 22]. The water content obtained automatically from the KF titrator. A blank determination using the same amount of acetone was used to correct the final water contents [19].
Solids content in the primary condensates were determined using the vacuum filtration technique suggested by Oasmaa and Peacocke [23]. The sample was filtered through a pre-dried and pre-weighed Whatman No. 2 qualitative filter paper with mean pore size of 8 µm. The filter paper with the retentate was then washed with excess amount of acetone until the filtrate became clear. The filter paper with the residue was air-dried for approximately 15 min and placed in an oven at 105 °C for 1 h, cooled in a desiccator and weighed. The drying, cooling and weighing steps were repeated until a constant weight was obtained.
CHNS analyses of the primary and secondary condensates were carried out using a CE-440 Carlo Erba Elemental Analyzer with ± 0.3% absolute accuracy [19]. In the procedure, the liquid samples were mixed with a known amount of acetone (1:6) to obtain the carbon, hydrogen and nitrogen (CHNS) contents. The CHNS composition of the organic fraction of the liquid product was calculated by subtracting the carbon, hydrogen and oxygen contents of the product water and the added acetone. Hence, the CHN data were obtained on dry, solvent-free basis. Oxygen content was determined by difference, using the percentage composition of CHN [24].
Compositional analyses of organic fractions of the liquid products (bio-oil) were performed using a PerkinElmer Clarus 680 GC–MS system [19]. The samples diluted with acetone were used for GC–MS analysis after filtration through a 0.2 µm pore size Sartorius filter. A sample volume of 1 µL was injected into the GC column via an injection port maintained at 300 °C, with 1:50 split ratio. The GC oven programme was initially held at 50 °C for 2 min, then ramped at 5 °C min−1 to 275 °C, and finally held at 275 °C for 3 min, giving a 50-min analysis time. Helium was used as carrier gas at a constant flow rate of 15 mL min−1. A column splitter was used to enable simultaneous detection of compounds separated on the columns by MS and FID detectors. Mass spectra were obtained using 70 eV ionisation energy in the molecular mass range of m/z = 35–300, with a scan time of 0.35 s. Assignments of the main peaks were made from mass spectral detection (NIST05 MS library). The detector temperature was 250 °C.
Characterization of Solid Residues
Solid residues obtained from these experiments were distributed into the bed material, char pot and liquid product obtained as primary condensates. However, in fast pyrolysis, the solid residue of interest is usually those found in the char pot and in this study, they represent over 90 wt% of the solid residues. Hence, only the characterization of the solid residue from the char pot was carried out and reported in this present study.
The ash content of solid residues obtained from the char pot was determined according to the ASTM D1762-84 method [25]. Approximately 4–5 g of solid residue was weighed into each pre-calcined and pre-weighed crucible set (crucible and lid) and placed in a furnace. The samples were ashed 750 °C for 6 h, followed by cooling in a desiccator to room temperature. After weighing, the ash content was then obtained by the difference in mass between the crucible + ash and the empty crucible. The average of five samples was taken to further reduce the deviation.
The solid residues were further analysed for CHNS composition using the same elemental analyzers used for the liquid products as described in “Characterization of Liquid Products” section. In addition, both the ash content bone-dry feedstock (2.69 wt%) and the solid residue obtained from its fast pyrolysis were analysed for other elements. The simple scan analysis was carried out using a Bruker S8 Tiger X-ray Fluorescence (XRF) spectrometer (University of Birmingham), which is capable of quantifying elements from sodium to uranium. Firstly, the samples were separately pulverised with mortar and pestle to make fine powders and weighed into a sample cup with Mylar window. The volume of each sample was approximately 1 cm3. The results of the elements with concentrations ≥ 0.02 wt% are reported in this work.
Determination of Heating Values of Pyrolysis Products
In this study, the calorific value of the liquid product was carried out only on the primary condensates, which had more organic product and less water content than the secondary condensates (please see Fig. 3 below). Approximately 1 g of the solvent-free primary condensate was burnt completely in an excess oxygen environment in a steel bomb calorimeter (Parr 6100 calorimeter) at constant volume. The same procedure was repeated for the solid products based on the ASTM D2015 Method [26]. The calorific values of gas product were estimated from the volume percentage of gas component and their calorific values (as higher heating values, HHV), according to Eq. 1.
$$HHV({\text{MJ}}~{\text{k}}{{\text{g}}^{ - 1}})=\mathop \sum \limits_{{i=1}}^{n} \left( {{x_i} \times C{V_i}} \right)$$
(1)
where i…n is the each combustible component in the gas product, x is mass fraction of combustible component in gas product, CV is the alorific value of combustible gas component in MJ kg−1.
Mass Balance Calculation
Mass balances (wt% on dry feed basis) were calculated by comparing the mass yields of final fast pyrolysis products (liquids, solid residues and non-condensable gases) with the mass of trommel fines feedstock processed. All metal, glassware items and transition pipes used in the bench-scale pyrolysis unit were weighed before and after each run, to calculate the yields of pyrolysis products. By difference in weight of trommel fines added to hopper before and trommel fines left in hopper after run, the amount of trommel fines fed can be calculated. The solid yield is a combination of the solid residue collected in the char pot (6), reactor (4) and metal transition pipe (7), and the solid fines or solids present in the tar derived liquid. The liquid product was fractionated into six fractions, which are (1) water condenser tar derived oil, (2) transition pipe 1, (3) dry ice acetone condenser 1, (4) transition pipe 2, (5) dry ice acetone condenser 2, and (6) cotton wool filter. 1 was taken as the primary condensate, while 2–6 was taken as the secondary condensate. In addition, the permanent gas yields were calculated based on the gas composition obtained from GC analysis, total gas volume and the exit gas temperature and the recorded pressure. The pyrolysis temperature was the average value of temperature data recorded by two K-type thermocouples at the fluidised-bed every 5 min throughout the run.